As commonly used in discussing polymeric fibers, “nanofibers” refers to fibers with “diameters” (i.e., maximum transverse cross-section dimension) less than 0.5 microns (i.e., 0.5×10−6 meters). Typical polymeric nanofibers have diameters between 50 and 300 nanometers (i.e., between 0.05×10−6 and 0.3×10−6 meters). Nanofibers provide for improved barrier fabrics for clothing and other applications, such as filtering. Only small quantities of nanofibers on the surface of meltblown fabrics greatly enhance liquid retention and decrease water contact angle. Other factors such as air resistance and breathability are also favorably impacted as nanofibers are added to a nonwoven fabric. These advantages notwithstanding, nanofibers have had limited commercial applicability, primarily because the production costs are too high.
The most common technique currently used for commercially producing nanofibers is electrospinning. In this technique a polymer is typically dissolved in a solvent (although polymer melts may also be used) and placed in a glass pipette tube sealed at an upstream end and having a small opening in a necked-down portion at the downstream end. A high voltage (>50 kV) is then applied between the polymer solution and a collector near the open downstream end of the pipette. This process can produce nanofibers with diameters as low as fifty nanometers, although the collected web usually contains fibers with varying diameters from fifty nanometers to two microns. The production rate of this process is very low and is typically measured in grams per hour, much too low to have wide commercial applicability. Moreover, the concentration of polymer in the solvent tends to be low (on the order of 10%) thereby further reducing the effective production rate and, if the system is operated at high volume, the operator is forced to contend with significant amounts of solvent and noxious off-gas byproducts. Further, switching the type of polymer that can be used in this process typically requires extensive machine modifications
It would be more desirable to use meltblown techniques to produce polymeric nanofibers. However, conventional meltblown webs have fiber diameters ranging from about one micron to ten microns. These webs are typically used for filtration applications, and the lowest possible fiber diameters are desirable because they offer better filtration efficiencies. Conventional meltblown spinning technology is limited to two microns because of the inability to make sufficiently small spin holes (i.e., spinning orifices). Typically, the spin hole diameters cannot be made smaller than approximately 0.005 inch and have an L/D (length over diameter) ratio of less than approximately 10.
An efficient use of meltblown technology is disclosed in U.S. Pat. No. 6,833,104 (Berger). The entire disclosure in that patent is expressly incorporated herein by reference. The spinneret disclosed in that patent includes a plate having channels etched or otherwise defined in a surface thereof, each channel having a downstream end extending to the edge of the plate. The plate surface is covered with a similarly configured plate (i.e., with correspondingly defined passages) or a flat plate to define closed flow passages, and the downstream ends serve as a lineal array of spinning orifices at the plate edge. Molten polymer is delivered to the upstream ends of the passages from a polymer source through a filter. The out-flowing polymer filaments or fibers flow parallel to the plate surfaces in which the spinning orifices are defined. For purposes of the description and claims herein, this type of spinneret will be referred to as a plate edge orifice spinneret (i.e., having plate edge spinning orifices in a linear array) to distinguish it from the spinneret type such as disclosed in U.S. Pat. No. 5,162,074 (Hills) in which the filaments are spun perpendicularly to the spinneret plate surface from which they emanate. Plate edge orifice spinnerets have been unable to produce fibers having diameters smaller than about 0.8 microns. It would be extremely valuable from a commercial perspective to be able to produce nanofibers from a meltblown process using plate edge orifice spinnerets with a commercially practical productivity rate.